In fact, iptables is more than just a firewall. You can use it to share your internet connection for your private network. If you have your internal network already working but you can't access internet, just add this rule to your gateway-pc iptables:

ferm is a tool to maintain complex firewalls, without having the trouble to rewrite the complex rules over and over again. ferm allows the entire firewall rule set to be stored in a separate file, and to be loaded with one command. The firewall configuration resembles structured programming-like language, which can contain levels and lists.

A good GUI based firewall, uses gnome dependencies. Firestarter has the ability to use both white and black lists for regulating traffic. Very simple and easy to use, with good documentation available on their website.

Really simple iptable configuration. (Nothing to do with gnome) Easy to configure, blocks everything not needed (almost) by default. Controlled by only one config file. It gave me all stealth on grc.com<br>

The Shoreline Firewall, more commonly known as "Shorewall", is high-level tool for configuring Netfilter. You describe your firewall/gateway requirements using entries in a set of configuration files. Shorewall reads those configuration files and with the help of the iptables utility, Shorewall configures Netfilter to match your requirements. Shorewall can be used on a dedicated firewall system, a multi-function gateway/router/server or on a standalone GNU/Linux system. Shorewall does not use Netfilter's ipchains compatibility mode and can thus take advantage of Netfilter's connection state tracking capabilities.